So many neighborhoods could be fixed by adding front/backyard ADUs, converting a handful of houses into commercial amenities tuned for the local community (cafes, small convenience/grocery stores, small libraries/coworking spaces, with minimal parking), and car-inaccessible passthroughs to nearby neighborhoods. There's way too much of a focus on building walkable communities, and not enough on converting existing ones with these small changes that don't disrupt the character in the way huge developments might (and in some cases might lend character to cookie-cutter sprawl that has none).

The problem is, converting an existing setup isn't easy. You just can't go and tear down houses that you got by using eminent domain - you need to either pony up a metric shit ton of money or you need to wait decades, no one will grant eminent domain to build a cafe and convenience store.

And even if you'd manage to acquire the property, you'd need to deal with zoning accomodation to allow non-residential use and that's where the NIMBYs will seriously throw wrenches wherever they can because it will mess with their property values.

Building from scratch doesn't have any of these associated efforts.

NIMBYs make new developments difficult to produce, too, actually, and for similar reasons: zoning and environmental laws. New developments also cost more, because you're putting in all-new infrastructure, whereas many old communities are approaching their replacement dates for water and sewer and whatnot anyway. You might as well get the most bang for your buck by introducing more avenues for generating property taxes (ADUs, subdivided lots, and especially commercial establishments).

Also, you don't need to tear anything down. Small neighborhood-use commercial establishments can be converted from existing housing.

Just to pick a random example in England:

https://www.google.com/maps/@52.5577247,-0.238575,3a,75y,287...

This is a small neighbourhood. There are all types of houses there. To your right is half of a regular semi-detached house having become a small pub, and its garage is a barbershop. The other half is still a normal house.

To your left, the two end houses on a terrace are joined to make a shop.

It adds immediate value to the neighbourhood, as the people who live there need walk no more than a few steps to get their milk and bread, to enjoy some social company in the evening, or to get their hair cut.

You'll also notice the density is nothing like US suburban houses with their masses of space all around each one. And if you travel a little further down the road, you'll see there are more shops not that far away!

Planning permission is handled by the local council, but it is mostly standardised. This would be conversion of usage class C3 (normal house) to A1 (shop) or A4 (pub). Councils have a list of things they're allowed to consider in planning applications (and solicit comments from the public for 21 days), called Material Considerations, and things they're not allowed to consider.

For example, they are allowed to consider traffic and parking, appearance of the area, noise and disturbance, loss of sunlight or daylight, etc. But they are not allowed to consider the effect on business or property values, or the reputation of the applicant.

https://en.wikipedia.org/wiki/Material_consideration

As you can see... if your country's laws support repurposing of property, then people will do it, and make a better world for themselves.

And the best part is that we don't even need to get this dense. Americans love our greenery, and there's space for that, just not multi-square-mile neighborhoods with NO commerce and housing only a few hundred (or a few dozen!) families. This is where planning IS helpful: identifying places to keep green-space, while also filling in dead-space.